1. Field of the Invention
The present invention relates to the treatment of wastewater solutions generally and, more particularly, to the treatment of such waste solutions containing hexavalent chromium so as to render the waste solutions safe for disposal.
2. Background Art
Waste solutions containing toxic amounts of hexavalent chromium are obtained in a variety of industrial operations, for instance metal stripping processes and electroplating processes. These waste solutions must be disposed of and, because of their hexavalent chromium content, serious problems were encountered in their disposal in the past, due to their toxicity to both aquatic and nonaquatic life. A conventional method of treatment is to treat these solutions with a reducing agent. When hexavalent chromium is reduced to trivalent chromium, the chromium is in a form which can be precipitated from solution in conventional treatment processes. Conventional methods of reducing hexavalent chromium include treatment with sodium metabisulfite, sulfur dioxide, or ferrous sulfate, in acidic solution.
The disadvantages of these conventional treatment methods are that solutions have to be made up from powdered materials, the solutions are generally corrosive, objectionable gases may have to be controlled, and/or materials are being added to the waste stream which will require later removal. The latter disadvantage is of particular importance, as it makes little sense to increase the burden on the overall treatment system.
It would, therefore, be desirable to employ a process for reducing hexavalent chromium in which no materials are added to the waste stream which need be removed later and in which process any added materials are relatively easy and safe to store and handle.
Frequently, particularly in waste streams from plating operations, cyanide and cyanide compounds are present in addition to hexavalent chromium. In my U.S. Pat. No. 3,505,217, issued Apr. 7, 1970, which is incorporated herein by reference, I show how free and combined cyanide in a waste solution can be treated with aldehyde or water-soluble bisulfite addition reaction product compounds of the aldehyde in basic solution to convert the cyanides to relatively harmless nitrogen compounds. While this method satisfactorily treats the cyanides as intended, the treatment may result in a residual concentration of aldehyde being present and, since the treatment takes place in basic solution, there can be no reduction of the hexavalent chromium which may be present.
Accordingly, it is a principal object of the present invention to provide a process for reducing hexavalent chromium without the introduction of materials which require later removal from a waste stream.
Another object of the invention is to provide a process for reducing hexavalent chromium which will also destroy residual aldehyde remaining from a previous cyanide treatment step.
Other objects of the invention, as well as particular features and advantages thereof will, be apparent from the following description.